Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers. The electrode layers typically comprise porous, electrically conductive sheet material as a substrate, and an electrocatalyst disposed at each membrane/electrode layer interface to induce the desired electrochemical reaction.
The electrocatalyst may be applied to the electrode substrate, or to the membrane electrolyte, using a variety of well-documented techniques. Typical fuel cell electrocatalysts are expensive. It is therefore important to use the electrocatalyst material as efficiently as possible. This includes increasing utilization of the electrocatalyst in the fuel cell electrodes.
In fuel cell operation, effective electrocatalyst sites are accessible to the reactant in the fuel cell, are electrically connected to the fuel cell current collectors, and are tonically connected to the fuel cell electrolyte. Electrocatalyst sites which are ionically isolated from the electrolyte are not productively utilized if ions generated by the fuel cell reactions at the electrocatalyst sites do not have a means for being ionically transported to the electrolyte.
A measure of electrochemical fuel cell performance is the voltage output from the cell for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output. Increasing effective utilization of the electrocatalyst enables the same amount of electrocatalyst to induce a higher rate of electrochemical conversion in a fuel cell resulting in improved performance.
U.S. Pat. No. 5,084,144 discloses an electrodeposition method for the preparation of fuel cell gas diffusion electrodes. A layer of proton-conducting polymer is impregnated into one surface of a carbon-containing electrode substrate. Electrocatalyst is then deposited on the surface of substrate using a DC or pulse current electrodeposition technique. Thus, together with a counterelectrode, the gas diffusion electrode is immersed in a bath containing primarily cations (M.sup.+, M.sup.++, M.sup.+++) of the metals of groups VIII or Ib of the periodic table, and a direct current is applied. The current may be constant or several current pulses with a relatively long pulse duration (duration of approximately 6-120 seconds) may be used. As a result, a thin layer of electrocatalyst is deposited only where it is ionically connected to the proton-conducting polymer coating, which in turn will be in contact with the electrolyte in the MEA.
Other electrodeposition processes have been used in the preparation of fuel cell electrodes. For example, in U.S. Pat. No. 5,599,638 and PCT/International Publication No. WO96/12317 (Application No. PCT/US94/11911), a controlled potential is used for the electrodeposition step. A constant voltage is applied for 5-10 minutes continuously, without current or voltage pulsing. This potentiostatic control permits some control of the electrochemical processes which occur, but the growth of the electrocatalyst clusters is difficult to control using the described technique. The substrate is not impregnated with any ion-conducting polymer prior to electrodeposition. Instead, the electrode is impregnated, preferably with a Nafion.RTM. or another proton-conductive polymer solution after electrodeposition of the electrocatalyst. Another article (Journal of Power Sources, 1998, 75, 230-235) discloses use of DC or pulsed current electrodeposition of electrocatalyst, again without pre-impregnation of the substrate with an ion-conducting polymer.
In certain of the electrodeposition processes described above, the electrode potentials are uncontrolled, and tend to vary at the surface of the electrodes according to the applied current and over time. Depending on the potential, different electrochemical processes and reactions will occur, as they are dependent on the overpotential. For example, some processes will not occur until the overpotential reaches a particular level. Where the potentials are controlled, a voltage is conventionally applied continuously resulting in relatively uncontrolled growth of electrocatalyst clusters during the electrodeposition process.
By controlling the potential and applying a pulsed voltage profile (varying voltage with time) between the electrodes (which is suited to the particular substrate and electrolyte) it is possible to selectively control the processes which will occur during the electrodeposition of an electrocatalyst, and to control the physical deposition of the electrocatalyst.